Published: January 18, 2007

Facing intense pressure from the Bush administration to show progress in securing Iraq, senior Iraqi officials announced Wednesday that they had moved against the country's most powerful Shiite militia, arresting several dozen senior members in the past few weeks.

It was the first time the Shiite government of Prime Minister Nuri Kamal al-Maliki had claimed significant action against the militia, the Mahdi Army, one of the most intractable problems facing his administration. The militia's leader, the cleric Moktada al-Sadr, helped put Mr. Maliki in power, but pressure to crack down on the group has mounted as its killings in the capital have driven a wedge into efforts to keep the country together.

Although the announcement seemed timed to deflect growing scrutiny by an American administration that has grown increasingly frustrated with Mr. Maliki, American officers here offered some support for the government's claims, saying that at least half a dozen senior militia leaders had been taken into custody in recent weeks.

In perhaps the most surprising development, the Americans said, none of the members had been prematurely released, a chronic problem as this government has frequently shielded Shiite fighters.

''There was definitely a change in attitudes,'' in the past three to four weeks, a senior American military officer said, speaking on condition of anonymity.

Mr. Maliki, in a meeting with foreign journalists on Wednesday, said 400 Mahdi militiamen had been arrested ''within the last few days.'' A senior government official said later by telephone that the total number arrested was 420 and that they had been detained in 56 operations beginning in October. Several dozen senior leaders have been detained in the past several weeks, the senior official said on condition of anonymity. He said the total number of senior commanders did not exceed 100.

Still, some American military officials remained skeptical that the effort was more than just a short-term attempt to appease them at a time when American government support for Mr. Maliki appeared to have sunk to an all-time low.

''The question is whether it will be sustained,'' another American military official said. ''This shouldn't be done to weather some short-term political storm. This has to stick in the long run.''

Whatever the case, changes have been felt on the street. In Shiite neighborhoods across the capital, militia members seem to have dropped from view in recent weeks, residents and militia members say. Shiite foot soldiers have tucked away their machine guns and have melted back into bustling city blocks, preparing for what they say they believe will be an American military onslaught against them.

''They have not run checkpoints for a week,'' said Ali, a merchant who lives in northern Baghdad and does business with the militia. ''They hid their weapons. They are bored.''

An influential Shiite sheik, Adel Ibrahim Subihawi, said of senior Mahdi members, ''They are making new passports right now to leave.''

It was not immediately clear whether the vanishing act was related to fear over the arrests or was a calculated move to wait out the coming American troop increase and prepare to re-emerge later.

But Iraqi and American officials acknowledge that the militia is the central challenge in the new effort to stop the cycle of violence in the capital, especially given the fighters' ability to move and hide freely in tightly knit Shiite neighborhoods.

It is a vast task ahead: American military intelligence officials have estimated that there are around 7,000 Mahdi militiamen in Baghdad. Many have split off into their own fiefs.

''They are like amoebas,'' one Shiite politician said. ''Always dividing and multiplying.''

American military commanders are now debating whether a large-scale offensive in Sadr City, a vast grid of cinder-block houses in northeastern Baghdad that is the stronghold of the Mahdi Army, should be the way to confront the fighters. American soldiers battled the militia twice in 2004, but it emerged stronger after Mr. Sadr entered the political system and helped install Mr. Maliki as Iraq's prime minister last spring.

One senior military official said a large-scale operation ''cannot be taken off the table,'' but others speak of plans to isolate the militia by cutting off northeastern neighborhoods where the militia dominates, and then continuing to chip away at its networks elsewhere.

In an interesting twist, the militia's leadership has not visibly fought back against the crackdown. American commanders say that the arrests do not draw the howling objections they used to in 2004, because Mr. Sadr's militia has splintered so deeply since then that the members they are arresting are more criminal than political and considered by Mr. Sadr to be disloyal renegades.

In that assessment, Mr. Sadr could even be using the government and the American military to purge his own ranks of undesirables.

Mr. Maliki, in the meeting on Wednesday, denied he was influenced by Mr. Sadr, and he offered as proof the fact that his government was finally taking painful action against his own Shiite constituency.